Novel ecological implications of non-toxic Microcystis towards toxic ecotype in population—promoting toxic ecotype dominance at various N levels and cooperative defense against luteolin-stress

Abstract

Abstract Microcystin (MC)-producing (MC+) and MC-free (MC−) Microcystis always co-exist and interact during Microcystis-dominated cyanobacterial blooms (MCBs), where MC+Microcystis abundance and extracellular MC-content (EMC) determine the hazard extent of MCBs. The current study elucidated intraspecific interaction between MC+ and MC−Microcystis at various nitrogen (N) levels (0.5–50 mg/L) and how such N-mediated interaction impacted algicidal and EMC-inhibiting effect of luteolin, a natural bioalgicide. Conclusively, MC+ and MC−Microcystis were inhibited mutually at N-limitation (0.5 mg/L), which enhanced the algicidal and EMC-inhibiting effects of luteolin. However, at N-sufficiency (5–50 mg/L), MC−Microcystis promoted MC+ ecotype growth and dominance, and such intraspecific interaction induced the cooperative defense of two ecotypes, weakening luteolin's algicidal and EMC-inhibiting effects. Mechanism analyses further revealed that MC+Microcystis in luteolin-stress co-culture secreted exopolymeric substances (EPSs) for self-protection against luteolin-stress and also released more EMC to induce EPS-production by MC−Microcystis as protectants, thus enhancing their luteolin-resistance and promoting their growth. This study provided novel ecological implications of MC−Microcystis toward MC+ ecotype in terms of assisting the dominant establishment of MC+Microcystis and cooperative defense with MC+ ecotype against luteolin, which guided the application of bioalgicide (i.e. luteolin) for MCBs and MCs pollution mitigation in different eutrophication-degree waters.